The Global System for Mobile Communication (GSM) cellular system has recently begun service using a new modulation standard referred to as Enhanced Data rates for GSM Evolution (EDGE). This standard uses 8-Level Phase Shift Keying (8PSK) modulation. Transmitters operating according to the EDGE standard must include a modulator providing both amplitude modulation and phase modulation. To efficiently amplify such modulation, a polar system is desirable.
Polar transmitters may be classified as either open loop or closed loop. In an open loop polar transmitter, a transmit signal is broken into amplitude and phase components. The phase component is provided to the radio frequency (RF) input of a power amplifier. The amplitude component may be used to vary the supply voltage, or collector voltage, of the power amplifier such that the output power of the power amplifier follows the instantaneous amplitude of the modulation envelope, thereby providing amplitude modulation.
One issue for an open loop polar transmitter is that amplitude modulation to phase modulation (AM/PM) distortion of the power amplifier varies as the supply voltage varies. One solution to this problem is to pre-distort the phase component prior to amplification in order to compensate for the AM/PM distortion of the power amplifier. However, if the Voltage Standing Wave Ratio (VSWR) at the output of the power amplifier changes due to variations in load impedance, then the AM/PM distortion also changes. As a result, the pre-distortion no longer aligns with the AM/PM distortion of the power amplifier. At low output power levels, this is generally not an issue because the spectrum and Error Vector Magnitude (EVM) requirements of the EDGE standard are less stringent for low output power levels. At higher output power levels, this may become more of an issue because the spectrum and EVM requirements are more stringent.
A closed loop polar transmitter may be used to solve the issue of AM/PM distortion by enclosing the power amplifier within a phase locked loop (PLL) generating the phase modulation. However, one issue for a closed loop polar transmitter is that strong interference signals present at the antenna are fed back to the PLL along with the output of the power amplifier. If the interference signal is strong enough, the interference signal will overpower the feedback from the output of the power amplifier and cause the PLL to unlock. As a result, the output of the polar transmitter is severely distorted.
Thus, there remains a need for a polar transmitter that provides improved spectral purity at higher output levels and that prevents unlocking of the PLL used to generate the phase component due to strong interference signals.